Colloidal nanoparticles are nanometer-sized solid particles that are of interest in many advanced industrial applications. Generally, colloidal nanoparticles include colloidal nanocrystals as well as amorphous particles, and can comprise either inorganic or organic solids. The term “nanometer-sized” is typically used to refer to particles with an approximate size range between about 1 nm to about 1000 nm in diameter (one nanometer is 10−9 meter). More typically, “nanometer-sized” refers to an approximate size range between about 1 nm to about 100 nm in diameter.
The “noble metals” include Cu, Ag, Au, Pt, Pd, and sometimes Ir, and are referred to as such because they are somewhat resistant to oxidation. Noble metal nanocrystals, including gold, silver, copper, and platinum, play a wide range of roles in diverse applications of science and technology, such as chemical catalysis, catalysts for the growth of nanowires, nanomedicines, nanoelectronics, and the like. Therefore, control of the noble metal nanocrystal size and size distribution are of great interest to enhance their utility in these applications.
Gold and other noble metal nanocrystals play important roles in many different branches of science and technology. For most applications, the size and size distribution control of nanocrystals are of key importance. At present, the Brust method (Brust, M., et al., Chem. Comm., 1994, 801–802), a two-phase approach, and its variations (Collier, C., et al., Science, 1997, 277:1978–1981; Leff, D. et al., Langmuir 1996, 12: 4723–4730; Cliffel, D. et al., Langmuir, 2000, 16: 9699–9702) are the most popular synthetic schemes in this field, although certain other approaches are available. (See, e.g., Turkevich, J. et al. Disc. Faraday Soc. 1951, 11: 55–75; Wallenberg, L. R., et al., Surf. Sci., 1985, 156: 256–64; Teranishi, T., et al., Adv. Mater., 1998, 10: 596–599; Green, M., et al., Chem. Commun., 2000, 183–184; Stoeva, S., et al., JACS, 2002, 124: 2305–2311; Murphy, C., et al., Adv. Mater., 2002, 14: 80–82) Among these methods, the citrate reduction method in aqueous solution (Turkevich, J. et al. Disc. Faraday Soc. 1951, 11: 55–75) has attracted significant attention. However, the nanocrystals can only be produced in a very low concentration, about 10–1000 times lower than that for the Brust method and its variations.
However, the size range of nanocrystals available from the Brust method is limited to between about 1 and about 4 nm, and the nanocrystal size distribution is also broad. Some post-synthesis treatments, such as ligand exchange (Brown, L. et al., JACS, 1999, 121: 882–883) and thermal annealing (Zhong, C. et al., Chem. Commun., 1999, 1211–1212; Lin, X. et al., Nanopart. Res., 2000, 2: 157–164; Shimizu, T., et al., J. Phys. Chem. B, 2003, 107: 2719–2724) are reported to improve the size distribution of gold (Au) nanocrystals formed through either the Brust method or other synthetic approaches. Another drawback of the Brust method is that the resulting nanocrystals are coated with a monolayer of strongly coordinating ligands, thiols, which makes it difficult to carry out the nanocrystals surface modification and functionalization needed for certain purposes.
In comparison to gold nanocrystals, synthesis of other noble metal nanocrystals is even less developed (Collier, C. et al., ibid; Ahmadi, T. et al., Science, 1996, 272: 1924–1926; Watzky, M., et al., JACS, 1997, 119: 10382–10400; Courty, A., et al., Adv. Mater., 2001, 13: 254–258; Ziegler, K., et al., JACS, 2001, 123: 7797–7803; Filankembo, A., et al., J. Phys. Chem. B, 2003, 107,:7492–7500).
In the patent literature, U.S. Pat. No. 6,645,444 (issued to Goldstein) reports copper nanocrystals having a particle size of 3 nm, but does not discuss the dispersity of the nanocrystals. The reported method entails dissolution of copper chloride and dodecylamine ligand in water. Hexane is layered onto the aqueous solution and some of the blue flocculent goes into the hexane phase. The two phase system is reduced with sodium borohydride in water to yield a reddish-brown hexane phase that contains the copper nanocrystals. This reference also reports that silver nanocrystals can be produced by a similar method.
U.S. Pat. No. 6,262,129 (issued to Murray et al.) describes a method of making nanoparticles under an inert atmosphere, which employs a phosphine ligand for the metal. U.S. Pat. No. 6,103,868 (issued to Heath et al.) reports the formation of functionalized Au, Ag, and Pt nanoparticles having a relatively narrow size distribution in the range of 1 to 20 nm. This reference employs a phase transfer agent capable of forming micelles, i.e., a bi-phasic method. U.S. Patent Publication 2004/0089101 reports making monodisperse copper nanocrystals that are passivated with a positively charged nitrogen-containing agent.
The formation of certain nearly monodisperse semiconductor nanocrystals has been reported by an organometallic approach (Murray, C., et al., JACS, 1993, 115: 8706–8715; Peng, X., et al., JACS, 1998, 120: 5343–5344) and alternative approaches (Peng, Z. et al, JACS, 2001, 123: 183–184; Qu, L., et al., Nano Lett., 2001, 1: 333–336; Yu, W. et al., Angew. Chemie Int. Ed., 2002, 41: 2368–2371). It is reported that one feature of this synthesis, as revealed by mechanism studies (Yu, W. et al., Chem. Mater., 2003, ASAP article), is maintaining balanced nucleation and growth by tuning the activity of the precursors. The knowledge learned in the synthesis of semiconductor nanocrystals is unlikely to be applicable for the Brust method because it is a bi-phasic process, for which both nucleation and growth occur at the interface of the two liquid phases.
Also, while significant progress has been made in the development of high quality and monodisperse nanocrystals of the cadmium chalcogenides, the quality of zinc chalcogenides nanocrystals using alternative approaches is significantly lower than that of cadmium chalcogenides nanocrystals. Further, the preparation of highly monodisperse and high quality magnetic nanocrystals such as iron oxide nanocrystals is of great interest for their applications in a variety of technological roles. The development of these materials has lagged behind the semiconductor materials.
An object of the present invention is to develop new methods to synthesize and stabilize, high quality, highly monodisperse, nanocrystalline materials. Monodisperse size distributions of noble metal nanocrystals are expected to exhibit useful optical, magnetic, electronic, or catalytic properties.